Within the processing industry, such as the chemical industry and the cellulose industry, it is common practice to deliver the chemicals to the factories in solid form or in the form of highly concentrated aqueous solutions. Prior to use, the concentration of the chemicals usually is lowered by diluting then with water. In for instance pulp mills, large quantities of concentrated sodium hydrate are used and in the industrial plants in such mills large quantities of low-energy vapour are produced, i.e. vapour having a poor heat content. The low-energy vapour usually is utilised to produce hot water. In modern pulp mills, increasingly higher temperatures are requested, particularly in the bleaching plants. No doubt it would be an advantage to be able to utilise low-energy vapour in the preparation of comparatively diluted chemical solutions and at the same time increase their temperature to a level allowing them to be used directly in the production process.
WO 91/18863 discloses a process for the recovery of lower aliphatic acids, such as formic acid, and sugars from the spent cooking liquor from a pulping process carried out with these acids. Spent cooking liquor is evaporated in several steps in a multiple effect evaporator wherein the vapour is returned to previous stages and heats and dilutes the cooking liquor. This publication is silent with ragard to the possibilitty of elevating the boiling points in order to recover maximum thermal energy from the system. Instead, the purpose of the invention described in that publication is to recover acids and sugars.
U.S. Pat. No. 4,755,258 describes multiple effect evaporation of spent liquor, wherein the formed vapour is restored to previous stages and heats the liquor. An advantage is said to be that the dry solids contents of the spent liquor remain nearly unchanged. The plant in accordance with this prior-art publication does not either make use of the boiling-point elevation to recover high-quality energy.